There is a rapidly expanding demand for fluoride based chemicals, both of the organic type such as fluorinated hydrocarbons, e.g., "Freon", and polytetrafluoroethylene resins, as well as for inorganic fluorides, such as aluminum fluoride and cryolite employed in the manufacture of metallic aluminum. The traditional raw material for these chemicals has been fluorspar, or calcium fluoride. Unfortunately, known reserves of high grade fluorspar ores have not expanded as rapidly as the demand for fluorine chemicals.
The only other potentially large sources of fluorine are the very extensive deposits of fluorapatite ore. This material is the principal constituent of the phosphate ores used in the manufacture of wet process phosphoric acid and phosphate fertilizers.
The art is replete with complex processes having the objective of recovering the fluoride values from these phosphate ores, either in the form of anhydrous hydrofluoric acid, or as aluminum fluoride. Representative of such art are U.S. Pat. Nos. 3,063,799; 3,024,086; 2,780,523; 2,588,786; 3,110,562; 3,219,410; 3,128,152; 3,195,979, 3,338,673; 3,101,254; 3,258,308; 3,218,128; 3,326,634; 3,256,061; 3,455,650; 3,511,603; and 3,316,000. Virtually all such processes use as a starting material fluosilicic acid, which has been recovered as a by-product of the manufacture of phosphoric acid and calcium phosphate fertilizers from the fluoride-containing phosphate ores.
Fluosilicic acid originates when strong mineral acids such as sulfuric, phosphoric, hydrochloric, or nitric acid are used to attack the fluorapatite ore, as is shown, for sulfuric acid, in equations (1) and (2) below: EQU (1) Ca.sub.10 F.sub.2 (PO.sub.4).sub.6 + 10 H.sub.2 SO.sub.4.fwdarw. 10 Ca(SO.sub.4) + 6 H.sub.3 PO.sub.4 + 2HF EQU (2) 6hf + siO.sub.2 .fwdarw.H.sub.2 SiF.sub.6 + 2H.sub.2 O
it should be noted that the hydrofluoric acid generated in reaction (1) reacts as shown in equation (2) to form fluosilicic acid. There is almost always sufficient silica associated as an impurity with the phosphate ore, that the hydrofluoric acid created is converted to fluosilicic acid under current operating procedures.
Many of the problems associated with the proposed recovery processes of the prior art stem from the characteristics of the fluosilicic acid starting material. Fluosilicic acid is volatile, highly corrosive, and toxic to plant, animal and marine life. Much of the cost of operation of current wet process phosphoric acid plants occurs as a result of these properties. Its volatility creates problems of the evolution of toxic and corrosive fumes which have to be trapped in scrubbers. Its toxic and corrosive character requires expensive waste disposal procedures. Expensive and exotic materials of construction are required for mechanical equipment such as pumps, tanks impellors, etc., used in the phosphate industry, because of the highly corrosive nature of process streams which contain fluosilicic acid.
Also the presence of fluosilicic acid limits the application of the wet process phosphoric acid product prepared from the phosphate ores. Such acid contains appreciable quantities, often 50% or more, of the fluoride originally present in the ore, and for this reason calcium or ammonium phosphates prepared from wet process phosphoric acid cannot be employed, for example, as animal feed supplements.
The prior art processes are often characterized by poor yields. This is in part due to the fact that only a portion of the fluorine in the ore is recoverable in the form of fluosilicic acid. Large percentages of the fluorine are found either in the waste streams or in the end product phosphoric acid and in fertilizers prepared from it.
The previously mentioned characteristics of fluosilicic acid also require that any process based upon recovery of fluoride values from fluosilicic acid be constructed of suitable equipment. For example, glass cannot be employed, since it is rapidly attacked, as is mild steel and even stainless steel. The volatility of H.sub.2 SiF.sub.6 also limits the range of processes which can be applied to it.
U.S. Pat. No. 3,619,136, discloses a process to preferentially dissolve the calcium and P.sub.2 O.sub.5 content of phosphate ores in a recycle phosphoric acid which is saturated with fluoride and with other minor impurities, to leave an insoluble residue of calcium fluoride, silica, and iron and aluminum phosphates. After sedimentation to remove this residue, the patent teaches reaction with sulfuric acid to precipitate the calcium as gypsum and the recovery of a fluoride-saturated phosphoric acid for recycle and as a product.